The frequency of myocardial infarction and sudden cardiac death has been shown to be higher in the hours from 6 a.m. to noon. Circadian variation in platelet aggregation has been hypothesized to contribute to this observation. We have developed a novel method of characterizing specific components of platelet activation in whole blood samples. Using flow cytometry to measure P-selectin expression on the surface of platelets, and fibrinogen binding we can look more specifically at platelet activation, as opposed to the measurement of platelet aggregation, that characterizes the summary of all components. This study is designed to characterize variation in platelet activation over a twenty-four hour interval.